INFLUENCE OF CATCHMENT TOPOGRAPHY ON WATER CHEMISTRY IN SOUTHEASTERN QUEBEC SHIELD LAKES

For 30 Canadian Shield lakes of southeastern Quebec, catchment slope a nd lake morphometry account for 50-70% of the variability of chlorophy ll a (Chi a), dissolved organic carbon (DOG), total phosphorus (TP), N O3-, and NH4+. Dissolved organic carbon, TP, Chi a, Ca, and Mg are neg atively related to catchment slope, whereas NO3- and NH4+ increase wit h increasing slope. Concentrations of more conservative constituents ( SO42-, Na, K) increase with decreasing elevation as a result of higher evapotranspiration and lower precipitation at low elevations. Catchme nt variables (slope, drainage area, percent wetlands) are as good pred ictors of Chi a (r(2)= 0.7) as are water chemistry variables (TP, Ca, Mg, and pH). Dominant vegetation (deciduous vs. coniferous) has little or no influence on lake water chemistry. Hydrogeological data for the Canadian Shield suggest that, during periods of high runoff, the deve lopment of waterlogged areas and the importance of overland flow on sa turated soils are inversely proportional to catchment slope. We propos e that the strong influence of catchment slope on water quality is due to slope-dependent seasonal waterlogging, which determines the fate ( retention or export to surface waters) of dissolved substances produce d within and moving through the forest floor.